Process of forming an aramid copolymer

ABSTRACT

The invention concerns processes for forming a polymer comprising residues of 2-(4-amino phenyl)-5 (6) amino phenyl benzimidazole (DAPBI), paraphenylene diamine, and terephthaloyl dichloride, comprising the steps of: a) forming a solution of DAPBI and paraphenylene diamine dihydrochloride in a solvent system comprising an organic solvent and an inorganic salt; b) cooling the solution to a temperature of 30° C. or less; c) adding a stoichiometric amount of terephthaloyl dichloride, relative to the total amount of the DAPBI and the paraphenylene diamine dihydrochloride, to the solution; and d) agitating the solution of step c) to form a polymer.

TECHNICAL FIELD

The present application concerns methods of producing aramid polymersderived from 5(6)-amino-2-(p-aminophenyl)benzimidazole (DAPBI),para-phenylenediamine dihydrochloride (PPD.2HCl) and terephthaloyldichloride (TCl) that are capable of forming fibers having superiorphysical properties.

BACKGROUND

Fibers derived from 5(6)-amino-2-(p-aminophenyl)benzimidazole (DAPBI),para-phenylenediamine (PPD) and terephthaloyl dichloride (TCl or T, alsocommonly referred to as terephthaloyl chloride) are known in the art.Such copolymers are the basis for a high strength fibers manufactured inRussia, for example, under the trade names Armos® and Rusar®. See,Russian Patent Application No. 2,045,586.

One issue in using DAPBI is that it has very poor solubility in aNMP/CaCl₂ solvent system. In order to prepare DAPBI/paraphenylenediamine (PPD) copolymers from copolymer solutions having higherconcentrations of polymer in the solvent system (i.e., above 4% weightpercent polymer in the solvent system), DAPBI is typically first fullydissolved in the solvent system by heating the combination to atemperature of about 70° C. or higher followed by addition of PPD. PPDdissolves readily in the solvent. The mixture of diamines in solution,however, then has to be cooled to lower temperature (i.e. ˜5° C.) beforestarting polymerization with TCl. DAPBI precipitates back out duringthis cooling process and polymerization becomes “heterogeneous”, withthe PPD remaining in solution and the DAPBI in solid state. It isbelieved the PPD in such a solution preferentially reacts with the TClupon its addition versus the DAPBI in its un-dissolved state, creatinghighly heterogeneous block polymers, with blocks of predominantly PPDand then blocks of predominantly DAPBI.

Another issue in using DAPBI is the two amines on DAPBI are verydifferent in reactivity and positional factors. The amine shown to theright in the below structure (the azole amine) is an order of magnitudemore reactive than the amine to the left in the structure (the benzylamine).

As a result, DAPBI/PPD copolymers prepared by a conventionalpolymerization process in a NMP/CaCl₂ solvent system tend to have nocontrol over the position of the monomer components. It is believed afactor in creating higher strength fibers from DAPBI/PPD copolymer isthe arrangement of the comonomers along the polymer chain. Inparticular, controlling the distribution of PPD and DAPBI comonomers isbelieved to help prevent the formation of crystal solvates in sulfuricacid solution and help the alignment of polymer chains during the heattreatment of fibers made from the copolymer, resulting in fibers withbetter mechanical properties.

SUMMARY

In some aspects, the invention concerns processes for forming a polymercomprising residues of 2-(4-amino phenyl)-5 (6) amino phenylbenzimidazole (DAPBI), paraphenylene diamine, and terephthaloyldichloride, comprising the steps of: (a) forming a solution of DAPBI andparaphenylene diamine dihydrochloride (PPD.2HCl) in a solvent systemcomprising an organic solvent and an inorganic salt; (b) cooling thesolution to a temperature of 30° C. or less; (c) adding a stoichiometricamount of terephthaloyl dichloride, relative to the total amount of theDAPBI and the paraphenylene diamine dihydrochloride, to the solution;and (d) agitating the solution of step c) to form a polymer.

In some embodiments, the organic solvent is N-methyl-2-pyrrolidone (NMP)or dimethylacetamide (DMAC). Suitable inorganic salts include LiCl andCaCl₂. In one preferred embodiment, the solvent system is NMP/CaCl₂. Incertain embodiments, the NMP/CaCl₂ solvent has a CaCl₂ weight percent inthe range of from 1 to 10%.

In some processes, in step a) the solution is formed by heating DAPBIand paraphenylene diamine dihydrochloride in the solvent system.Terephthaloyl dichloride can be added stepwise or in a single step.

The molar ratio of DAPBI to phenylene diamine can be in the range offrom 0.25 to 4.0 for some reactions. The amount of the solution that isDAPBI is step (a) is in the range of from 0.3 to 9.0 weight % in certainreactions. The amount of solution that is paraphenylene diaminedichloride in step (a) can be in the range of from 0.2 to 10.0 weight %.

The polymer can be isolated in some embodiments. The polymer can betreated with one or more washing steps, neutralizing steps, or both. Insome embodiments, the polymer can be comminuted. The washing and/orneutralizing steps can be performed before or after comminuting thepolymer.

In some aspects, the invention additionally concerns the step ofdissolving the polymer in a solvent comprising sulfuric acid to form asolution suitable for spinning fibers. The polymer to be dissolvedincludes the isolated polymer that may or may not have been washedand/or neutralized and polymer may or may not have been comminuted.

In another aspect, the invention concerns polymer comprising residues of2-(4-amino phenyl)-5 (6) amino phenyl benzimidazole (DAPBI),paraphenylene diamine (or paraphenylene diamine dihydrochloride), andterephthaloyl dichloride, the polymer having a IPC peak block ratio(Relative Elution Time of sample/DAPBI-T homopolymer) of 1.43 to 1.47,preferably 1.44-1.46, and an inherent viscosity of greater than 2 dl/g.In certain embodiments, the polymer has an inherent viscosity of 4 dl/gor greater.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description, isfurther understood when read in conjunction with the appended drawings.For the purpose of illustrating the invention, there is shown in thedrawings exemplary embodiments of the invention; however, the inventionis not limited to the specific methods, compositions, and devicesdisclosed. In the drawings:

FIG. 1 presents IPC results from the examples versus that of DAPBI-Thomopolymer.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In some aspects, the invention concerns processes for forming a polymercomprising residues of 2-(4-amino phenyl)-5 (6) amino phenylbenzimidazole (DAPBI), paraphenylene diamine, and terephthaloyldichloride, comprising the steps of: (a) forming a solution of DAPBI andparaphenylene diamine dihydrochloride (PPD.2HCl) in a solvent systemcomprising an organic solvent and an inorganic salt; (b) cooling thesolution to a temperature of 30° C. or less; (c) adding a stoichiometricamount of terephthaloyl dichloride, relative to the total amount of theDAPBI and the paraphenylene diamine dihydrochloride, to the solution;and (d) agitating the solution of step c) to form a polymer.

In the present invention when PPD.2HCl rather than PPD is used in thehot solution of DAPBI in NMP/CaCl₂, upon cooling to ˜5° C., thedissolved DAPBI did not precipitate with the resultant cooled solutionof diamines being a homogeneous solution. When the PPD.2HCl is usedrather than the PPD, the color of solution becomes “blue”, and while notwanting to be bound by theory, it is believed that this reflects thepreferential formation of a hyper-conjugated and delocalized electronicstructure of DAPBI by accepting HCl from PPD.2HCl, as shown below. Inother words, it is believed that the blue color is the result of theplanar DAPBI.HCl.

The planar structure restricts the rotation around a single bond inDAPBI and helps to provide higher chain rigidity. This homogeneoussolution of DAPBI and PPD is then polymerized with TCl, providing a highinherent polymer.

In some embodiments, the solvent or solvent containing the diamines iscooled, or the reaction mixture is cooled, to the lowest temperature thesystem can withstand (such as the freezing point of the solvent, etc.)to avoid side reactions between the TCl and the solvent. In someinstances, it is useful to cool the solvent or solvent mixture to 10degrees C. or less, preferably 5 degrees C. or less. Vessels useful forproducing polymers, and temperatures and other conditions useful inproducing polymers, include, for example, details disclosed in suchpatents as U.S. Pat. No. 3,063,966 to Kwolek et al.; U.S. Pat. No.3,600,350 to Kwolek; U.S. Pat. No. 4,018,735 to Nakagawa et al.; andU.S. Pat. No. 5,646,234 to Jung et al.

In some embodiments, the organic solvent is N-methyl-2-pyrrolidone (NMP)or dimethylacetamide (DMAC). Suitable inorganic salts include LiCl andCaCl₂. In one preferred embodiment, the solvent system is NMP/CaCl₂. Incertain embodiments, the NMP/CaCl₂ solvent has a CaCl₂ weight percent inthe range of from 1 to 10%. It should be noted that the solubility ofCaCl₂ in NMP is about 8%. As such, when more than 8% CaCl₂ is used, someundissolved CaCl₂ is present in the solvent system. The solvents andsalts can be obtained from commercial sources and, if desired, purifiedby methods known to those skilled in the art.

The amount of the solution that is DAPBI is step (a) is in the range offrom 0.3 to 9.0 weight % in certain reactions. The amount of solutionthat is paraphenylene diamine dichloride in step (a) can be in the rangeof from 0.2 to 10.0 weight %.

The polymer can be isolated in some embodiments. The isolated polymercan be comminuted a desired particle size to assist in processing andstorage. The polymer can be treated with one or more washing steps,neutralizing steps, or both. These washing and/or neutralizing steps canbe performed before or after comminuting the polymer. Equipment suitablefor use in agitation of the reaction mixtures, washing andneutralization steps, and comminuting the polymer is known to thoseskilled in the art.

Molecular weights of polymers are typically monitored by, and correlatedto, one or more dilute solution viscosity measurements. Accordingly,dilute solution measurements of the relative viscosity (“V_(rel)” or“η_(rel)” or “n_(rel)”) and inherent viscosity (“V_(inh)” or “η_(inh)”or “n_(inh)”) are typically used for monitoring polymer molecularweight. The relative and inherent viscosities of dilute polymersolutions are related according to the expression

V_(inh)=In (V_(rel))/C,

where In is the natural logarithm function and C is the concentration ofthe polymer solution. V_(rel) is a unitless ratio, thus V_(inh) isexpressed in units of inverse concentration, typically as deciliters pergram (“dl/g”).

Neutralization of the polymer can occur in one or more steps bycontacting the polymer with a base. Suitable bases include NaOH; KOH;Na₂CO₃; NaHCO₃; NH₄OH; Ca(OH)₂; K₂CO₃; KHCO₃; or trialkylamines,preferably tributylamine; other amines; or mixtures thereof. In oneembodiment, the base is water soluble. In some preferred examples theneutralization solution is an aqueous solution of base.

The polymer can also be washed with water independent of or prior toand/or after the neutralization step.

In some aspects, the invention additionally concerns the step ofdissolving the polymer in a solvent comprising sulfuric acid to form asolution suitable for spinning fibers (also referred to as a “spindope”). The polymer to be dissolved includes the isolated polymer thatmay or may not have been washed and/or neutralized and polymer may ormay not have been comminuted. While any suitable solvent can be used todissolve the polymer, in some embodiments the solvent comprisesN-methyl-2-pyrrolidone (NMP) or dimethylacetamide (DMAC) and aninorganic salt to form a solution suitable for spinning fibers. Thedissolved polymer can be spun into fibers by conventional techniquesknown to those skilled in the art.

The spin dope containing the copolymer described herein can be spun intodope filaments using any number of processes; however, wet spinning and“air-gap” spinning are the best known. The general arrangement of thespinnerets and baths for these spinning processes is well known in theart, with the figures in U.S. Pat. Nos. 3,227,793; 3,414,645; 3,767,756;and 5,667,743 being illustrative of such spinning processes for highstrength polymers. In “air-gap” spinning the spinneret typicallyextrudes the fiber first into a gas, such as air and is a preferredmethod for forming filaments

As used herein, the terms filaments and fibers are used interchangeably.

The fiber(s) can be contacted with one or more washing baths orcabinets. Washes can be accomplished by immersing the fiber into a bathor by spraying the fiber with the aqueous solution. Washing cabinetstypically comprise an enclosed cabinet containing one or more rolls thatthe yarn travels around a number of times, and across, prior to exitingthe cabinet. As the yarn travels around the roll it is sprayed with atleast one washing fluid. The washing fluid is continuously collected inthe bottom of the cabinet and drained therefrom.

The temperature of the washing fluid(s) is preferably greater than 30°C. The washing fluid can also be applied in vapor form (steam), but ismore conveniently used in liquid form. Preferably, a number of washingbaths or cabinets are used. The residence time of the yarn in any onewashing bath or cabinet will depend on the desired amount of residualsulfuric acid left in the yarn. In a continuous process, the duration ofthe entire washing process in the preferred multiple washing bath(s)and/or cabinet(s) is preferably no greater than about 10 minutes, morepreferably greater than about 5 seconds. In some embodiments theduration of the entire washing process is 20 seconds or more; in someembodiments the entire washing is accomplished in 400 seconds or less.In a batch process, the duration of the entire washing process can be onthe order of hours, as much as 12 to 24 hours or more.

If needed, neutralization of the acid (such as sulfuric acid solvent) inthe yarn can occur in a bath or cabinet. In some embodiments, theneutralization bath or cabinet can follow one or more washing baths orcabinets. Washes can be accomplished by immersing the fiber into a bathor by spraying the fiber with the aqueous solution. Neutralization canoccur in one bath or cabinet or in multiple baths or cabinets. In someembodiments, preferred bases for the neutralization of sulfuric acidimpurity include NaOH; KOH; Na₂CO₃; NaHCO₃; NH₄OH; Ca(OH)₂; K₂CO₃;KHCO₃; or trialkylamines, preferably tributylamine; other amines; ormixtures thereof In one embodiment, the base is water soluble. In somepreferred examples the neutralization solution is an aqueous solutioncontaining 0.01 to 1.25 mols of base per liter, preferably 0.01 to 0.5mols of base per liter. The amount of cation is also dependent on thetime and temperature of exposure to the base and the washing method. Insome preferred embodiments, the base is NaOH or Ca(OH)₂.

After treating the fiber with base, the process optionally can includethe step of contacting the yarn with a washing solution containing wateror an acid to remove all or substantially all excess base. This washingsolution can be applied in one or more washing baths or cabinets.

After washing and neutralization, the fiber or yarn can be dried in adryer to remove water and other liquids. One or more dryers can be used.In certain embodiments, the dryer can be an oven that uses heated air todry the fibers. In other embodiments, heated rolls can be used to heatthe fibers. The fiber is heated in the dryer to a temperature of atleast about 20° C. but less than about 100° C. until the moisturecontent of the fiber is 20 weight percent of the fiber or less. In someembodiments the fiber is heated to 85° C. or less. In some embodimentsthe fiber is heated under those conditions until the moisture content ofthe fiber is 14 weight percent of the fiber or less. The inventors havediscovered that low temperature drying is a preferred route to improvedfiber strength. Specifically, the inventors have found that the bestfiber strength properties are achieved when the first drying step (i.e.heated roll, heated atmosphere as in an oven, etc.) experienced by thenever-dried yarn is conducted at gentle temperatures not normally usedin continuous processes used to dry high strength fibers on commercialscale. It is believed that the copolymer fiber has more affinity towater than PPD-T homopolymer; this affinity slows the diffusion rate ofwater out of the polymer during drying and consequently if thenever-dried yarn is directly exposed to typical high dryingtemperatures, generally used to create a large thermal driving force andreduce drying time, irreparable damage to the fiber occurs resulting inlower fiber strength. In some embodiments, the fiber is heated in thisfirst drying step to at least to about 30° C.; in some embodiments thefiber is heated in this first drying step at least to about 40° C.

The dryer residence time is less than ten minutes and is preferably lessthan 180 seconds. The dryer can be provided with a nitrogen or othernon-reactive atmosphere. The drying step typically is performed atatmospheric pressure; however, if desired, the step can be performedunder reduced pressure. In one embodiment, the filaments are dried undera tension of at least 0.1 gpd, preferably a tension of 2 gpd or greater.

The invention also concerns a polymer powder comprising residues of2-(4-amino phenyl)-5 (6) amino phenyl benzimidazole (DAPBI),paraphenylene diamine (PPD) (or paraphenylene diamine dihydrochloride(PPD.2HCl)), and terephthaloyl dichloride, capable of being dissolved inN-methyl-2-pyrrolidone (NMP) or dimethylacetamide (DMAC) and aninorganic salt. It should be noted that polymers made by traditionalprocesses, once isolated, do not have the capability of beingredissolved in a solvent system of NMP or DMAc and an inorganic salt butrequire a solvent such as sulfuric acid to achieve a spinning solution.In some embodiments, the polymer powder has an inherent viscosity ofgreater than 2 dl/g. In some preferred embodiments, the polymer has aninherent viscosity of 4 dl/g or greater.

In some embodiments, the polymer is characterized by having a IPC peakblock ratio (Relative Elution Time of sample/DAPBI-T homopolymer) of1.43 to 1.47, preferably 1.44-1.46, when run under the conditionsdiscussed in the description of the IPC test detailed below.

Definitions

As used herein, the term “residue” of a chemical species refers to themoiety that is the resulting product of the chemical species in aparticular reaction scheme or subsequent formulation or chemicalproduct, regardless of whether the moiety is actually obtained from thechemical species. Thus, a copolymer comprising residues of paraphenylenediamine (or paraphenylene diamine dihydrochloride) refers to a copolymerhaving one or more units of the formula:

Similarly, a copolymer comprising residues of DAPBI contains one or moreunits of the structure:

A copolymer having residues of terephthaloyl dichloride contains one ormore units of the formula:

Dashed lines indicate bond positions.

The term “polymer,” as used herein, means a polymeric compound preparedby polymerizing monomers, whether of the same or a different type. Theterm “copolymer” (which refers to polymers prepared from two differentmonomers), the term “terpolymer” (which refers to polymers prepared fromthree different types of monomers), and the term “quadpolymer (whichrefers to polymers having four different types of monomers) are includedin the definition of polymer.

The term “powder” when referring to polymer means particles of thecopolymer having neither fibrous qualities like fiber or pulp, norfibrous film-like qualities like fibrids. Individual particles tend tobe fibril-free, have a random shape, and an effective particle diameterof 840 micrometers or less. U.S. Pat. Nos. 5,474,842 & 5,811,042 areillustrative.

As used herein, “stoichiometric amount” means the amount of a componenttheoretically needed to react with all of the reactive groups of asecond component. For example, “stoichiometric amount” refers to themoles of terephthalic didchloride needed to react with substantially allof the amine groups of the amine component (paraphenylene diaminedihydrochloride and DAPBI). It is understood by those skilled in the artthat the term “stoichiometric amount” refers to a range of amounts thatare typically within 10% of the theoretical amount. For example, thestoichiometric amount of terephthalic dichloride used in apolymerization reaction can be 90-110% of the amount of terephthalicacid theoretically needed to react with all of the paraphenylene diamineand DPABI amine groups.

The “fiber” refers to a relatively flexible, macroscopically homogeneousbody having a high ratio of length to width across its cross-sectionalarea perpendicular to its length. In preferred embodiments, the fiber isupon inspection essentially solid in cross-section, having few randomvoids or open areas that would be considered defects in the fibers.

The term “organic solvent” is understood herein to include a singlecomponent organic solvent or a mixture of two or more organic solvents.In some embodiments, the organic solvent is dimethylformaaide,dimethylacetamide (DMAC), N-methyl-2-pyrrolidone (NMP), ordimethylsulfoxide. In some preferred embodiments, the organic solvent isN-methyl-2-pyrrolidone or dimethylacetamide.

The term “inorganic salt” refers to a single inorganic salt or to amixture of two or more inorganic salts. In some embodiments, theinorganic salt is sufficiently soluble in the solvent and liberates anion of a halogen atom having an atomic number in the solvent. In someembodiments, the preferred inorganic salt is KCl, ZnCl₂, LiCl or CaCl₂.In certain preferred embodiments, the inorganic salt is LiCl or CaCl₂.

By “never-dried” it is meant the moisture content of the fiber is atleast 75 weight percent of the fiber.

As used in the specification including the appended claims, the singularforms “a,” “an,” and “the” include the plural, and reference to aparticular numerical value includes at least that particular value,unless the context clearly dictates otherwise. When a range of values isexpressed, another embodiment includes from the one particular valueand/or to the other particular value. Similarly, when values areexpressed as approximations, by use of the antecedent “about,” it willbe understood that the particular value forms another embodiment. Allranges are inclusive and combinable. When any variable occurs more thanone time in any constituent or in any formula, its definition in eachoccurrence is independent of its definition at every other occurrence.Combinations of substituents and/or variables are permissible only ifsuch combinations result in stable compounds.

Test Methods

Inherent viscosity can be determined using a solution in which a polymeris dissolved in a concentrated sulfuric acid with a concentration of 96wt % at a polymer concentration (C) of 0.5 g/dl and at a temperature of25° C. Inherent viscosity is then calculated as In (t_(poly)/t_(solv))/Cwhere t_(poly) is the drop time for the polymer solution and t_(solv) isthe drop time of the pure solvent.

Interaction Polymer Chromatography (IPC) method was used to analyze themicrostructure of p-aramid copolymers. The chromatographic separationswere performed on Alliance 2695™ Separation Module with column heater at60° C. from Waters Technologies (Milford, Mass., USA). The moduleprovides a low-pressure quaternary gradient pumping system with lagvolume 0.6 ml to the column outlet, online solvent degassing andautomatic sample injection from 2-ml vials. Waters UV/Vis 487™photometer at 320 nm wavelength was used as an online detector. Twocomponents of a mobile phase used were water (component A) andN,N′-dimethylformamide (DMAc) with 4% lithium chloride (LiCl) (componentB). 20 minutes linear gradient from 70% B to 100% B at flow rate 0.5ml/min was used for separation. Silica NovaPak™ 150×3.9 mm with 60 Apore size from Waters was used as a stationary phase.

Each sample was dissolved in DMAc with 4% LiCl at 120° C. with moderateagitation for 12 hours at concentration 0.2 mg/ml, and injected using 10ml injection loop.

Empower™ version 2 software module from Waters was used for dataacquisition and reduction. The IPC peak block ratio is then determinedby running a data set including a control of DAPBI-T homopolymer and thecopolymer to be evaluated. The Figure is an typical illustration of thepeaks for the homopolymer and copolymers being evaluated, with thevertical arrows indicating the peak values. The IPC peak block ratio isthen calculated by the following formula:

IPC peak block ratio=Minutes to peak for copolymer/Minutes to peak forhomopolymer.

The invention is illustrated by the following examples, which are notintended to be limiting in nature.

Examples

NMP, DMAC, LiCl, CaCl₂, DAPBI, PPD and TCl were obtained from commercialsources.

Example 1

To 1 liter reaction kettle equipped with basket stirrer, nitrogeninlet/outlet, 83.71 grams of NMP/CaCl₂ premix (8.3 wt % (weight ofsalt/total weight of salt plus solvent)), 161.65 grams of NMP(N-methyl-2-pyrrolidone), 12.282 grams (0.055 moles) of DAPBI(5(6)-amino-2-(p-aminophenyl)benzimidazole), 4.250 grams (0.023 mole) ofPPD.2HCl (p-phenylenediamine dihydrochloride) were added and stirred atroom temperature. Unlike the case of PPD addition, all of the solidmaterials were completely dissolved within 30 minutes of stirring atroom temperature. The clear solution showed light bluish color. Thereaction mixture was cooled to below 10° C. by stirring in a ice-waterbath. At this point, 5.560 grams (0.027 moles) of TCl was added all atonce and stirred for 5 minutes under nitrogen. Ice-water bath wasremoved and 10.325 grams of 2^(nd) TCl was added all at once and stirredslowly for 30 seconds and then the stirrer speed was increased to themaximum. The solution became very viscous, gelled and finally became arubbery mass. The rubbery mass hardened with time and broke into smallchunks of irregular sizes. The mixture was stirred for 30 more minutes.The resulting polymer was transferred to Waring® Blender and was groundto small particles and washed several times to remove solvent(NMP/CaCl₂) and HCl generated from both PPD.2HCl and the polymerizationreaction. Then the polymer was neutralized with sodium bicarbonate andfinally washed with water a few times to get neutral polymer. Thepolymer was transferred into a tray and dried at 120° C. over night in avacuum oven with nitrogen sweep. The polymer inherent viscosity was 7.36dl/g as measured by dissolving in sulfuric acid and measuring per thetest method.

Comparison Example

This example illustrates the preparation of DAPBI/PPD-T copolymer inNMP/CaCl₂ solvent by the conventional way of adding TCl to the solutionof both DAPBI and PPD in the solvent.

83.71 grams of NMP/CaCl₂ premix (8.3 wt % (weight of salt/total weightof salt plus solvent)), 163.32 grams of NMP, and 2.538 grams [0.023moles] of PPD were added to a 1-liter reaction kettle equipped with abasket stirrer and a nitrogen inlet/outlet. The mixture was stirred atroom temperature until all PPD is dissolved completely in the solvent.Then 12.282 grams (0.055 moles) of DAPBI was added and stirred for 15more minutes at room temperature. Unlike the Example above, the solutionlooked milky due to undissolved DAPBI. The solution was the mixture ofcompletely dissolved PPD and partially dissolved DAPBI, a heterogeneoussystem. The mixture was cooled to below 10° C. by placing in ice waterbath and stirred for 15 minutes. 5.573 grams (0.027 moles) of TCl wasadded and stirred for 5 minutes. The ice-water bath was removed and10.351 grams of a second portion of TCl was added all at once andstirred. The solution became very viscous and gelled within 4 minutesand the stirring continued for another 25 minutes. Highly viscous lumpsof polymer were transferred to a Waring® blender and were ground tosmall particles and washed several times to remove solvent (NMP/CaCl₂)and excess HCl generated by the reaction. Then the polymer wasneutralized with sodium bicarbonate and finally washed with water a fewtimes to get neutral polymer. The polymer was transferred into a trayand dried at 120° C. overnight in a vacuum oven with nitrogen sweep. Thepolymer inherent viscosity was 5.47 dl/g as measured by dissolving issulfuric acid and measuring per the test method.

Determination of IPC Values

IPC peak block ratios were determined for the polymer of Example 1 andthe Comparison Example by the test method described above. Results arepresented below.

Peak Value Peak Ratio Homopolymer (Control) 18.00 min NA Example 1 26.29min 1.46 Comparison 27.16 min 1.50

What is claimed:
 1. A process for forming a polymer comprising residuesof 2-(4-amino phenyl)-5 (6) amino phenyl benzimidazole (DAPBI),paraphenylene diamine, and terephthaloyl dichloride, comprising thesteps of: a) forming a solution of DAPBI and paraphenylene diaminedihydrochloride in a solvent system comprising an organic solvent and aninorganic salt; b) cooling the solution to a temperature of 30° C. orless; c) adding a stoichiometric amount of terephthaloyl dichloride,relative to the total amount of said DAPBI and said paraphenylenediamine dihydrochloride, to the solution; and d) agitating the solutionof step c) to form a polymer.
 2. The process of claim 1, wherein theorganic solvent is N-methyl-2-pyrrolidone (NMP) or dimethylacetamide(DMAC).
 3. The process of claim 1, wherein the inorganic salt is LiCl orCaCl₂.
 4. The process of claim 1, wherein the solvent system isNMP/CaCl₂.
 5. The process of claim 1, further comprising isolating saidpolymer from said solvent system.
 6. The process of claim 1, whereinstep a) said solution is formed by heating DAPBI and paraphenylenediamine dihydrochloride in said solvent system.
 7. The process of claim1, wherein terephthaloyl dichloride is added stepwise.
 8. The process ofclaim 1, wherein terephthaloyl dichloride is added in a single step. 9.The process of claim 5, further comprising the step of comminuting thepolymer.
 10. The process of claim 5, further comprising treating thepolymer with one or more washing steps, neutralizing steps, or both. 11.The process of claim 9, further comprising treating the polymer with oneor more washing steps, neutralizing steps, or both.
 12. The process ofclaim 10, further comprising the step of dissolving the polymer in asolvent comprising sulfuric acid to form a solution suitable forspinning fibers.
 13. The process of claim 11, further comprising thestep of dissolving the polymer in a solvent comprising sulfuric acid toform a solution suitable for spinning fibers.
 14. The process of claim1, wherein DAPBI and phenylene diamine are present in a molar ratio inthe range of from 0.25 to
 4. 15. The process of claim 4, whereinNMP/CaCl₂ has a CaCl₂ weight percent in the range of from 1 to 10%. 16.The process of claim 1, wherein the amount of DAPBI used in forming thesolution in step (a) is in the range of from 0.3 to 9.0 weight %. 17.The process of claim 1, wherein the amount of paraphenylene diaminedichloride used in forming the solution in step (a) is in the range offrom 0.2 to 10.0 weight %.
 18. The process of claim 1, wherein thesolution is cooled to a temperature of 10° C. or less.